Excitation-contraction coupling in the heart bears certain similarities to that of other striated muscle but is unique in other features. To assess some of the reasons for this, isolated sarcoplasmic reticulum (SR) fractions from dog cardiac muscle will be compared with those of pure fast twitch and pure slow twitch and rabbit back muscles. Subfractions of specific regions of SR will be purified and examined for functional capacity (CaATPase activity and characteristics, Ca sequestration, drug responsiveness) as well as morphological (freeze fracture and freeze drying electron microscopy) and physical (protein content and species, phospholipid content and species) characteristics. Effects of ligands, temperature, drugs and functional state will be compared by freeze fracture in isolated SR as well as intact muscle. Purification of CaATPase will be carried out from each subfraction of the various native SR preparations (longitudinal elements, junctional couplings, i.e., terminal cisternae and subsarcolemmal couplings) to infer whether these regions may play different roles in the excitation-contraction cycle. Substitution of phospholipids (and cholesterol) of known headgroup and fatty acyl composition will enable us to monitor the effects of phospholipid moieties on CaATPase as well as transport of Ca. The role of phospholipid charge and fatty acyl chain characteristics on dynamics of the membranes (CaATPase and Ca transport) and on membrane fluidity and function will be examined and compared for all fractions.